GitLab oauth requires Google settings

Nobody has claimed this yet.

I set up gitlab oauth this way:
```
- name: TOKEN_SECRET
value: "MY_GENERATED_TOKEN"
- name: GENERIC_NAME
value: "gitlab"
- name: GENERIC_CLIENT_ID
value: "MY_GITLAB_CLIENT_ID"
- name: GENERIC_CLIENT_SECRET
value: "MY_GITLAB_CLIENT_SECRET"
- name: GENERIC_AUTH_URL
value: "https://gitlab.FOOBAR.com/oauth/authorize"
- name: GENERIC_TOKEN_URL
value: "https://gitlab.FOOBAR.com/oauth/token"
- name: GENERIC_SCOPES
value: "api,openid,read_user"
- name: PUBLIC_URL
value: "https://chronograf.FOOBAR.com"
- name: GENERIC_API_URL
value: "https://gitlab.FOOBAR.com/api/v3/user"
```
when container started i see error in logs:
```
time="2020-06-01T14:41:01Z" level=error msg="Failed to validate Oauth settings: missing Google oauth setting[s]: client id, client secret" basepath=invalid component=server
```
If i add this two envs (GOOGLE_CLIENT_ID and GOOGLE_CLIENT_SECRET) with fake values, then chronograf started successfully and i see login screen with two buttons - google and gitlab

Reproduce the container startup with the listed GENERIC_* GitLab OAuth variables and inspect the validation path that emits “missing Google oauth setting[s]”. Confirm that GitLab-only configuration starts without fake GOOGLE_CLIENT_ID and GOOGLE_CLIENT_SECRET values, while preserving the intended Google login behavior.

Written by the indexing model from the issue text.